Liquid crystal devices (LCDs) typically comprise a thin layer of liquid crystal material contained between a pair of cell walls. The internal surface of the cell walls are usually coated with a certain material, or are suitably adapted in some way, to impart a degree of surface alignment to the liquid crystal. The bulk of the liquid crystal then adopts a configuration that depends on the surface alignment properties of the cell walls and on various other factors, such as the type of liquid crystal material and the thickness of the liquid crystal layer. Optically transparent electrode structures on one or both of the cell walls allow an electric field to be applied to the liquid crystal layer.
A typical liquid crystal display device is designed such that two, or more, liquid crystal configurations can be selected by the application of suitable electric fields. The different liquid crystal configurations are designed to be optically distinguishable so that optical contrast can be attained from the liquid crystal device. For example, a liquid crystal device suitably arranged between a pair of polarisers may have one configuration that will allow transmission of light through the system and a second configuration that will prevent it.
Monostable liquid crystal devices, in which the liquid crystal molecules can only adopt one stable configuration, are known. Application of an electric field can distort the configuration of the liquid crystal molecules, but once the electric field is removed the liquid crystal will relax back to its single stable configuration after some characteristic time (typically tens of milliseconds to a few seconds).
Twisted nematic (TN) and super-twisted nematic (STN) LCDs are examples of monostable devices. The TN and STN devices may be switched to an “on” state by application of a suitable voltage, and will switch back to an “off” state when the applied voltage falls below a certain level. It should be noted that the terms “on” and “off” relate to application of high (i.e. switching) voltage and low (i.e. non-switching) voltage respectively not necessarily the observed optical transmission of a display. As these devices are monostable, loss of power leads to loss of the image.
Multi pixel TN and STN passive matrix displays can be constructed using stripes of row and column electrodes on the upper and lower cell surfaces which allows the device to be multiplexed. The drive voltages applied to the row and column electrodes are selected such that a number of separate RMS voltage levels may be applied to each pixel of the display. The optical transmission of a typical TN device varies non-linearly with RMS voltage in a threshold transition manner as detailed by Alt and Pleschko in IEEE Trans ED 21 1974 pages 146–155. The maximum number of pixels that can be addressed using RMS methods is dictated by the optical transmission versus voltage characteristics of the TN or STN device, and in practice it has proven difficult to produce passive matrix STN displays with significantly more than about 500 lines of information due to cross-talk effects and manufacturing tolerances. It has also been demonstrated that other orthogonal functions, such as Walsh functions, can be used to passively address TN and STN displays.
Incorporating a driving element, such as thin film transistor (TFT) or other such non-linear elements (e.g. back-to-back diode, ferroelectric layers etc), adjacent to each pixel in a TN LCD has been found to significantly increase the total number of pixels which can be addressed; such displays are termed active matrix. In addition to the increased number of pixels that can be incorporated in a TN display, active matrix TN devices have many other advantages compared with multiplexed TN or STN displays; such as a relatively low operating voltage requirement, a wide temperature operating range and the capability to provide greyscale. The speed with which TFTs can be switched allows rapid sequential scanning through the row and columns in a display thereby permitting high speed, video rate, operation. As each pixel is isolated using the TFT, the effect of cross-talk is effectively removed in active matrix devices.
Many monostable devices permit grey-scale to be attained by controlling the magnitude of voltage applied to the layer of liquid crystal material. Selecting a TN liquid crystal configuration that has a shallow transmission versus voltage characteristic, and applying a drive voltage to produce an intermediate transmission level, allows greyscale to be attained in active matrix TN devices. A storage capacitor may also be included with the pixel which allows the retention of charge, and hence maintains the electric field across the liquid crystal for a limited period of time. In a TN active matrix device the liquid crystal is monostable, and as the electric field across the pixel decays it will return towards its relaxed configuration. Therefore, maintenance of an image on an active matrix TN display requires the regular updating of each pixel and hence a continual supply of power.
Active matrix TN devices are commonly used today in commercial displays for laptop computers, computer monitors, portable TV etc and devices are known to those skilled in the art which can operate with multiple levels of greyscale at video frame update rates. More detailed reviews of active matrix LCD technology have been prepared; see, for example, R. G. Stewart (1996) Active Matrix LCD, Society for Information Display, Seminar Lecture Notes, Volume 1, 13th May 1996 San Diego Convention Centre, Calif., M5-1-35.
The use of semiconductor circuits transferred to the device substrates using printing techniques or fluidic self-assembly also allow the formation of discrete semiconductor circuits which are capable of addressing two or more display pixels. A review of such devices is given in R. G. Stewart, (2000) Proceedings of the 20th International Displays Research Conference, p415–418, held at Palm Beach Fla. USA, 25–28 Sep. 2000.
U.S. Pat. No. 06,120,588 described how electro-phoretic inks can also be used in conjunction with TFTs. The use of the TFT active matrix removes cross-talk effects from the electro-phoretic ink, which does not have a significant threshold between its states. These devices are also described in K. Amundsen and P Drzaic (2000), Proceedings of the 20th IDRC, 84–87.
Another known type of LCD device is a bistable device. In a bistable LCD, the liquid crystal material can adopt two different, and stable, configurations in the absence of an applied electric field. Research into bistable LCDs has been prompted mainly by their inherent ability to store images and high multiplexibility. This negates the need for devices that have expensive active matrix back-planes and permits line at a time passive addressing.
Application of suitable electric fields to the bistable liquid crystal layer causes switching between the two stable configuration in which it can exist; so called “latching”. Hereinafter “latching” shall be taken to mean changing the liquid crystal from one stable configuration to another stable configuration such that the state remains after the applied voltage is removed, whilst “switching” shall mean any field induced change to the configuration of the liquid crystal which includes monostable switching effects.
Bistable liquid crystal devices are almost exclusively addressed using passive matrix techniques; the displays are constructed using stripes of row and column electrodes on the upper and lower cell surfaces which allows the device to be multiplexed. The inherent ability to store images in the absence of power allows potentially complex images to be built up a line at a time and also makes bistable devices attractive in applications where low power consumption is required, for example in laptop computers, PDAs and mobile telephone devices.
Examples of bistable liquid crystal displays include surface stabilised ferroelectric liquid crystal (SSFLC) devices as described by N A Clark and S T Lagerwall, Appl. Phys. Lett., 36, 11, 899 (1980). Ferroelectric liquid crystal device are generally passively addressed but, because of the extremely rapid speed in which FLC materials can latch between the two stable configurations, they have also been combined with active matrix backplanes to produce devices having very fast frame times; see, for example, see J. Xue and M. A. Handschy, (2000) Proceedings of the 20th IDRC, p13–17.
It has also been demonstrated previously (see U.S. Pat. No. 5,604,616) that it is possible to operate a cholesteric display in a pseudo bistable manner. The display can be electronically latched from a first stable state to a second stable state by application of a high voltage. Once the device is latched into the second state it is effectively “frozen” in that state and it is only possible to reliably reselect the first stable state by heating the device above the isotropic temperature of the liquid crystal material. The first and second stable states are twisted nematic configurations that possess a different degree of overall twist. Operating the device in what is termed the nematic mode permits RMS switching of device from either of the stable states. The device of U.S. Pat. No. 5,604,616 thus provides none of the benefits of truly bistable operation. For example, such a device does not allow images that persist in the absence of applied electrical power to be written and subsequently rewritten many time using only electrical addressing techniques.
It has also been shown in Patent Applications WO 91/11747 (“Bistable electrochirally controlled liquid crystal optical device”) and WO 92/00546 (“Nematic liquid crystal display with surface bistability controlled by a flexoelectric effect”) that a nematic liquid crystal can adopt, and can be switched between, two stable states via the use of chiral ions or flexoelectric coupling.
WO 97/14990 teaches how a zenithally bistable device (ZBD) may be constructed using a grating of a given design such that nematic liquid crystal molecules can adopt two stable pretilt angles in the same azimuthal plane. One of these states is a high pretilt state, whilst the other is a low pretilt state and a device is described which can adopt, and can be readily switched between, either of the two stable liquid crystal configurations. WO99/34251 teaches another ZBD device having a negative dielectric anisotropy material in a twisted nematic configuration. Patent application GB0017953.1 describes a zenithally stable device exhibiting multi-stability rather than bistability.
The two stable liquid crystal configurations of ZBD persist after driving electrical signals have been removed, and (see Wood et. al. SID Digest 2000) the device is highly resistant to mechanical shock, provides 10s of microsecond latching times at low driving voltages (<20V) and allows a high degree of multiplexibility. It has also been shown, see Bryan-Brown et al, (1998) proceedings of Asia Display, p1051–1052, that grey-scale may be achieved using a chirped grating that allows partial switching of a pixel.
Although bistable devices are ideal for low power and low cost application, there is also a requirement in certain applications (such as when displaying video images) to have a number of levels of grey in-between the dark and light states. Greyscale can be achieved in truly bistable devices using temporal and/or spatial dither where the perception of grey-levels is provided by switching each pixel “on” and “off” at a rate faster than the viewer can perceive or by dividing each pixel into two or more weighted sub-pixel regions.
Employing spatial and/or temporal dither techniques in bistable display devices does however increase the complexity, and hence unit cost, of devices. For example spatial dither increases the number of row and column drivers, requires thinner tracks thereby increasing track resistance and resistive powers losses in the panel and also requires more accurate etching to ensure linearity of the greyscale response. It is for these reasons, that passively addressed bistable devices known to those skilled in the art are, for the present at least, somewhat limited in there ability to produce high numbers of grey-levels and moving video images.
It is an object of this invention to mitigate some of the disadvantages associated with the liquid crystal devices described above.